1. Field of the Invention
The present invention relates generally to vehicle load floors of sandwich-type composite structure having a cellular core and, in particular, to such load floors whose structure is reinforced locally.
2. Background Art
Referring to the drawings and, in particular, to FIG. 1, a prior art underbody assembly 10 is shown for a motor vehicle, generally indicated at 12, in accordance with U.S. Pat. No. 6,126,219. The motor vehicle 12 includes a pair of body side assemblies 14 operatively connected to sides of the underbody assembly 10. It should be appreciated that only one body side assembly 14 is illustrated.
The motor vehicle 12 includes a front end assembly 16 operatively connected to a forward or front end of the underbody assembly 10. The motor vehicle 12 also includes a pair of rear cargo doors 18 operatively adjoined to a rear or back end of the underbody assembly 10. It should be appreciated that the motor vehicle 12 includes a roof (not shown) and other closures (not shown) which are also common for the two load floor heights. It should also be appreciated that, except for the underbody assembly 10, the motor vehicle 12 is conventional and known in the art.
The underbody assembly 10 includes an underbody load floor or floor pan 20 extending longitudinally and transversely. The underbody assembly 10 also includes a side sill 22 extending longitudinally along each side of the floor pan 20. The side sill 22 has a top surface 24 and a bottom surface 26 and a ledge 28 extending longitudinally and disposed between the top surface 24 and the bottom surface 26. For a high underbody load height for a rear-wheel drive configuration of the motor vehicle 12 illustrated in FIGS. 1 and 3, the floor pan 20 is operatively connected to the top surface 24 of the side sills 22 by suitable means such as welding. It should be appreciated that the side sills 22 are used along the load floor to body side interface along the edges of the floor pan 20 to take up the different position of the floor pan 20 relative to the body side assemblies 14.
The underbody assembly 10 includes a transition structure, generally indicated at 30, operatively connected between the floor pan 20 and the front end assembly 16. The transition structure 30 includes a pair of side members 32 extending longitudinally and transversely spaced. The side members 32 are operatively connected to front rails (not shown) of the front end assembly 16 by suitable means such as welding. The transition structure 30 also includes a floor pan extension 34 extending transversely and disposed on an upper surface of the side members 32. The floor pan extension 34 is generally planar and operatively connected to the side members 32 by suitable means such as welding. It should be appreciated that the floor pan extension 34 is used to make a transition between the floor pan 20 and a front floor 35 of the front end assembly 16 (which is fixed at the same position for both the high and low underbody load floors forward of a B-pillar (not shown) to maintain the position of a driver of the motor vehicle 12). It should also be appreciated that the transition structure 30 is used to achieve the different underbody load floor heights for the high and low underbody load floors to accommodate the different heights of the rear relative to the front of the motor vehicle 12.
The underbody assembly 10 may include a pair of extensions 36 positioned on the bottom of the rear side members 37 connected to the floor pan 20 and a rear sill (not shown) for the high underbody load floor of FIGS. 1 and 3 to maintain the size of a rear door opening, allowing the use of the common rear doors 18. The extensions 36 extend longitudinally and are operatively connected to the rear side members 37 by suitable means such as welding. The extensions 36 allow the rear side members 37 to make a transition to the rear door opening.
Such a load floor or floor pan as noted above includes substantial structural reinforcements, such as metal bars or tubes, to meet the load requirements. Such structure is typically very heavy and manufacturing costs and complexity are high, therefore improvements are desirable.
Sandwich-type materials having cellular cores have very important characteristics resulting from their being light in weight yet very rigid.
Conventionally, such a panel is constructed by sandwiching a cellular core having low strength characteristics by gluing it or bonding it between two skins, each of which is much thinner than the cellular core but has excellent mechanical characteristics.
The patent document FR 2 711 573 discloses a method of making a panel of sandwich-type composite structure having a cellular core. In that method, said panel is made in a single step by subjecting a stack to cold-pressing in a mold, which stack is made up of at least a first skin made of a stampable reinforced thermoplastics material, of a cellular core made of a thermoplastics material, of a second skin made of a stampable reinforced thermoplastics material, and of a first external covering layer made of a woven or non-woven material, the skins being preheated outside the mold to a softening temperature.
Such a method is particularly advantageous because of the fact that it makes it possible, in a single operation, both to generate cohesion between the various layers of the composite structure, and to shape the panel.
The resulting panel conserves all of the mechanical properties imparted by the cellular core sandwich structure.
European patent EP 0 649 736 B1 explains the principle of molding substantially flat parts out of thermoplastic sandwich material (TSM). The part is made in a single stage by pressing in a cold mold, at a pressure in the range of 10 bars to 30 bars, a stack consisting of at least a first top skin layer of stampable reinforced thermoplastics material, a cellular or honeycomb core of thermoplastics material and a second bottom skin layer of stampable reinforced thermoplastics material. The axes of the cells of the cellular core are generally oriented perpendicular to the skin layers. The skin layers and core are previously heated outside the mold to a softening temperature. Such sandwich material is also described in U.S. Pat. No. 5,683,782. The cellular core of such material enables the part to be very rigid while being light in weight.
U.S. Pat. No. 6,050,630 discloses a molded composite stack including a cellular core for a vehicle and a mold for forming the stack into a vehicular part, such as a floor panel.
Panels of sandwich-type composite structures having a cellular core have strength characteristics sufficient to enable mechanical structures subjected to large stresses to be reinforced structurally without making them too heavy. Such panels are in common use in shipbuilding, aircraft construction, and rail vehicle construction.
However, the non-uniformness of the mechanical stresses to which they are subjected sometimes makes it necessary to form local reinforcing plies at those places in said panels where the mechanical stresses are greatest.
In the field of aircraft construction, sandwich structure composite panels are made that are based on thermosettable resins reinforced with glass fibers.
In order to impart the desired shapes to the panels, and to maintain the shapes, the glass fibers and the thermosettable resin (in the form of pre-impregnates) are deposited layer-by-layer in a mold, and are then heated to high temperatures so as to cure (i.e. polymerize) the resin permanently.
The molds used may have a punch or a die, or else both a punch and a die.
Making such locally-reinforced panels consists firstly in defining zones where stresses are concentrated in the resulting panels, such zones being defined either by real testing or by computer simulation, and then in adding reinforcing plies at those places so as to make it possible to withstand such stresses.
The reinforcing plies are one-directional mats or woven fabrics of glass fibers, of carbon fibers, or of natural fibers embedded in a thermosettable resin, with an orientation that is determined by the orientation of the stresses. They are cut out to a pattern using special machines, e.g. water-jet cutting machines.
The reinforcing plies are disposed layer-by-layer in a mold, either manually or by means of a robot, with each ply having its own orientation.
That operation may be referred to as the “laying up” operation.
Then comes the baking step which is the longest step of the method of making such pieces because the stack of layers must be heated sufficiently to cure the thermosettable resin.
The various layers disposed in the mold are pressed in the mold by evacuating the mold. Such evacuation serves to press the materials against the die or the punch, and to remove surplus resin.
The desired shape is thus obtained with the fibers being impregnated with the resin as well as possible.
That “lamination” technique, and in particular the “laying up” operation, is characterized by a very low level of automation, and a large labor input.
Although, by means of the concept of localizing the strength, that technique makes it possible to achieve performance levels that are high for the pieces that are made in that way, it requires rigorous monitoring of quality.
As a result, that technique is very costly and cannot be used at the high production throughputs implemented in many fields such as the automobile industry.
U.S. Pat. No. 6,136,259 discloses a carpeted load floor for a vehicle which is blow molded and has an internal cavity.
Plastics processing technology has enjoyed significant recent advances, such that traditional high-strength materials such as metals are being replaced with fiber composite materials. These materials are not only light, but also are flexible and durable.
U.S. Pat. Nos. 5,891,560 and 6,165,604 disclose fiber-reinforced composites prepared from a depolymerizable and repolymerizable polymer having the processing advantages of a thermoset without being brittle. Impregnation of polymer into the fiber bundle is achieved, while still producing a composite with desirable physical properties and high damage tolerance.
Other load floors or floor pans are shown in the following U.S. patents: U.S. Pat. Nos. 6,179,362; 6,065,795; 6,053,566; 6,045,174; 6,209,205; 6,170,905; 6,128,815; 6,039,351; and 6,036,252.
As noted in the above-mentioned '560 and '604 patents, although thermoset composites have excellent mechanical properties, they suffer from several disadvantages: thermoset matrices have relatively limited elongation, the thermoset precursors are a source of undesirable volatile organic compounds (VOCs), the composites cannot be reshaped or recycled, and their production rates are limited.
Consequently, in principle at least, thermoplastic composites would solve many of the problems associated with thermosets. For example, unlike thermosets, thermoplastics can be reshaped, welded, staked, or thermoformed. Furthermore, thermoplastics are generally tougher, more ductile, and have greater elongation than thermosets.